In the field of medicine, phantoms or simulators are very useful. Certain sub-anatomical features present a development/manufacturing challenge. For example, brain ventricles are negative spaces within the brain that allow cerebral spinal fluid (CSF) to circulate and nourish the brain and provide protection against compression. A number of currently used methods to prepare these structures have challenges. One method of creating voids or negative spaces involves using balloons. The balloons are inflated within a liquid hydrogel which is then cooled. After a freeze-thaw cycle (FTC), the balloon is removed from the hydrogel. This remove leaves a scar on the surface and may require post-processing such as back-filling with water and removal of air pockets that develop.